The optical properties of eumelanin (from Sepia officinalis) are found to vary with particle size. The absorption spectrum for small eumelanin particles agrees quantitatively with the reported action spectra for photoinduced oxygen consumption and free radical generation by eumelanin. These small particles, unlike the large ones, generate long-lived reactive intermediates upon absorption of UV light. The data presented suggest that the small eumelanin particles may be involved in UV-A-induced photochemical processes believed to lead to DNA damage in skin cells, whereas the large particles efficiently dispose of UV-A energy through rapid nonradiative decay processes. These results provide new insight into the dichotomy that eumelanin is both photoprotective and photosensitizing. This size-dependent photoreactivity may be one of the contributing factors to the observed variations in skin cancer rates among different skin types.

Scanning electron microscopy (SEM) is used to examine the structure of natural and synthetic melanins. Eumelanin from Sepia officinalis and synthetic eumelanin are found to be structurally dissimilar. The natural sample has a significant structural order with subunits that have a lateral dimension of approximately 15 nm. The synthetic samples, on the other hand, appear to be amorphous solids. These results lend support for the existence of fundamental structural units proposed from the analyses of wide-angle X-ray diffraction measurements and previous mass-spectrometry results. These findings also provide insight into the disparate photophysical behavior of Sepia and synthetic eumelanin.

The emission properties of eumelanin from Sepia officinalis are examined following UV-A excitation. The emission decay is nonexponential, exhibiting decay components on the tens of picosecond to several nanosecond time scales. The corresponding depolarization dynamics are also nonexponential and reveal that the emission becomes totally depolarized with an average time constant of ∼80 ps at 20 °C. The depolarization of the emission is found to be activated; a simple Arrhenius fit to the depolarization rate data gives an activation barrier of 21 ± 3 kJ mol-1. The nonexponential emission decay is concluded to be a reflection of the structural disorder of eumelanin. The rapid and nonexponential depolarization dynamics are attributed to energy transfer processes that occur within “spherical” subunits that comprise the eumelanin aggregates.